Documentation: gpio: clarify sysfs line values are logical

Clarify that line values are logical, not physical, by replacing high/low
terminology with active/inactive.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Reviewed-by: Andy Shevchenko <amdy.shevchenko@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>